This invention relates to arrangements and methods for providing digital services to telephone subscribers via the subscriber loop.
In a conventional telephone system, the majority of subscribers are connected to local exchanges via twisted conductor pairs, generally referred to as subscriber loops. Between the subscribers and the exchange, the subscriber loops are carried in cables each containing a large number of conductor pairs. The cables issuing from the exchange feed smaller street cables from which the individual subscriber loops are xe2x80x98droppedxe2x80x99 to provide the final link to the subscriber. These twisted pair subscriber loops, were originally installed to carry voice services, but are now being used by the system operators to carry digital services such as ADSL (asymmetric digital subscriber line) in the frequency spectrum above the baseband frequencies used for the voice or POTS (plain ordinary telephone service) traffic. Typically, a duplex service such as ADSL is launched on to the subscriber loop at each end and may carry e.g. video and/or Internet traffic to the subscriber.
The currently employed ADSL service is a broad band technology which occupies a frequency band above that of voice to provide high bit rate (asymmetric) services to customers. Typically this service uses discrete multi-tone technology (DMT) at frequencies from as low as 26 kHz up to about 1.1 MHz at a downstream bit rate of 2 Mb/s or multiples thereof. For example, bit rates of 6 Mb/s may be used over short distances. A particular concern of service providers is the freedom to introduce new digital services that do not destabilise existing services and can operate in parallel with those services that are currently in use. Specifically, there are now proposals to introduce VDSL (very high bit rate digital subscriber line) services primarily for business customers to carry large volumes of data traffic. These services will typically use frequencies above 1.2 MHz, i.e. above the upper frequency limit for current ADSL services. It is a particular concern of the various Standards Bodies who ultimately control the widespread introduction of services that any new service should not disturb currently established services or compromise bandwidth that might be required in the future.
In order that system operators can be persuaded to introduce the a VDSL service to business subscribers, they will thus need assurance that this service introduction will have no adverse effect on their existing ADSL subscribers and, conversely, that the VDSL services can operate successfully in the presence of ADSL traffic. The latter is essential to the successful introduction of VDSL to ensure that a high quality reliable service is delivered to the customer.
We have found that when attempts are made to provide these two services to subscribers from an exchange, mutual interference can arise between subscriber lines in the transmission path from the exchange. The exchange is fed by large multi-pair cables in which the lines of a large number of subscribers are bundled together, and interference, generally referred to as cross-talk, can occur when the lines of two subscribers receiving respectively the two services are disposed adjacent each other in such a feeder cable. One solution to this problem is to provide physical separation of the subscriber lines carrying the two different services by bundling the VDSL subscriber lines into a separate feeder cable thus providing complete isolation of this traffic. This is a simple and effective solution, but in practice it can prove costly as it involves a significant investment in new cabling and/or reallocation of conductor pairs before the service can be offered to subscribers. A further effective solution to the problem is to increase the spectral guard band between the services to a level at which the interference becomes negligible. This however is costly in bandwidth which is a strictly limited and valuable commodity and is thus unattractive to the system operators.
It is an object of the invention to minimise or to overcome this disadvantage.
A further object of the invention is to provide an improved arrangement and method for the delivery of digital services to a telephone subscriber.
A further object of the invention is to provide an arrangement and method for providing ADSL and VDSL services to telecommunications customers via a common exchange or switch.
According to a first aspect of the invention there is provided a local telecommunications network system arranged to provide first and second services in respective first and second frequency bands to system subscribers, the system comprising an exchange or switch to which the subscribers are coupled each via a respective subscriber loop, means for applying said first service to selected subscriber loops, means for applying said second service to other selected subscriber loops, first filter means associated with said selected subscriber loops, and second filter means associated with said other subscriber loops, wherein each said filter means is arranged to provide a band edge cut-off of its respective first or second frequency band so as to inhibit interference of service-carrying signals between said first and second frequency bands. According to a further aspect of the invention there is provided a local telecommunications network system arranged to provide a first asymmetric digital subscriber line (ADSL) service in a first frequency band to a first set of subscribers and a second very high bit rate digital subscriber line (VDSL) service in a second frequency band higher than said first frequency band to a second set of subscribers, the system comprising an exchange or switch to which the subscribers are coupled each via a respective subscriber loop, first line card means for applying said ADSL service to selected subscriber loops, second line card means for applying said VDSL service to other selected subscriber loops, first low-pass filter means associated with said first line card means, and second high-pass filter means associated with said second line card means, wherein each said filter means is arranged to provide a band edge cut-off of its respective first or second frequency band so as to inhibit interference of service-carrying signals between said first and second frequency bands.
According to a further aspect of the invention there is provided a local telecommunications network system arranged to provide a first asymmetric digital subscriber line (ADSL) service in a first frequency band to a first set of subscribers and a second very high bit rate digital subscriber line (VDSL) service in a second frequency band higher than said first frequency band to a second set of subscribers, the system comprising an exchange or switch to which the subscribers are coupled each via a respective subscriber loop, first line card means for applying said ADSL service to selected subscriber loops, second line card means for applying said VDSL service to other selected subscriber loops, a main distribution frame associated with the exchange and whereby each subscriber loop is coupled to its respective line card means, each said subscriber loop having said ADSL service being further coupled via a POTS splitter and the main distribution frame to a further telephony line card, first low pass filter means associated with said first line card means, and second high pass filter means associated with said second line card means, wherein each said filter means is arranged to provide a band edge cut-off of its respective first or second frequency band so as to inhibit interference of service-carrying signals between said first and second frequency bands.
According to another aspect of the invention there is provided a method of providing first and second services in respective first and second frequency bands to subscribers in a local telecommunications network system comprising an exchange or switch to which the subscribers are coupled each via a respective subscriber loop, first line card means for applying said first service to selected subscriber loops, second line card means for applying said second service to other selected subscriber loops, the method comprising low pass filtering traffic on subscriber loops carrying said first service, and high pass filtering traffic on subscriber loops carrying said second service so as to inhibit interference of service-carrying signals between said first and second frequency bands.
We have found that by providing low pass filtering of the ADSL traffic and high pass filtering of the VDSL traffic, the two types of traffic can co-exist within a multi-pair cable without the risk of mutual interference. This allows the introduction of VDSL services at low cost to the system operator and without the need for additional cabling expense.
The low pass and high-pass filters may comprise balanced-T inductor/capacitor circuits.
Advantageously, the low pass and high pass filtering can be applied via a notch filter of common design.
In a preferred embodiment, the filtering is applied to the subscriber line card at the exchange.